1. Field of the Invention
This invention relates to a process for preparing an unsaturated glycol diester from a conjugated diene compound. More particularly, it is concerned with an improved process for preparing the desired 1,4 diester in high conversion and selectivity by reacting a conjugated diene, a carboxylic acid and oxygen in the presence of a solid catalyst in the liquid phase.
2. Description of the Prior Art
It is known in the prior art to prepare an unsaturated glycol diester, such as 1,4-diacetoxy-2-butene by reaction of butadiene, acetic acid and oxygen in the presence of a solid catalyst, such as palladium containing one or more metals as promotors. Such a process is described in U.S. Pat. No. 3,755,423; U.S. Pat. No. 3,742,039; U.S. Pat. No. 3,723,510; U.S. Pat. No. 3,671,577; German Pat. No. 2,354,218; German Pat. No. 2,419,921; German Pat. No. 2,417,658 and German Pat. No. 2,415,248. The diester product then can be readily hydrogenated and hydrolyzed to 1,4-butanediol, an important industrial solvent and chemical intermediate.
A major disadvantage of the prior processes is that the activity or selectivity, or both, of the catalyst in the reaction is poor, and accordingly, high conversion of the diene into the desired product gives considerable amounts of undesirable by-products.
Accordingly, a need exists for a catalyst system which is capable of producing the desired product at high conversion rates without an increase in unwanted by-products. Of particular importance is the need to provide a catalyst which can promote the desired conversion of butadiene to 1,4-diacetoxy-2-butene or the corresponding 3,4 product which can be readily isomerized to 1,4, but without forming in any significant amount the 1,3 isomer simultaneously, the latter being incapable of isomerization into the desired 1,4 product.